The trade-off between depth of focus and transverse resolution is fundamental to classic imaging systems. The primary solution with such classic systems has been to take multiple images while changing the focal plane to achieve high resolution in multiple planes. One good example of this is the confocal microscope, where the focus of the light is stepped in the axial direction to acquire a series of high resolution en-face images that are combined to create a 3D volume.
Recently, holoscopic systems have demonstrated the ability to generate depth invariant transverse resolution, where the resolution at all depths is equal to the resolution at the beam focus, by combining the angle diverse out-of-focus light from multiple adjacent acquisitions (see for example, swept source based full-field holoscopy system discussed in Hillmann, D. et al., Opt. Express 20, 21247-63, 2012, and swept source based line-field holoscopy system discussed in US Patent Publication No. 2014/0028974, each of which are hereby incorporated by reference). However, point scanning holoscopic systems have still been limited to the resolution given by the numerical aperture (NA) of the illumination beam and collection system. Here, the NA of the collection system is defined as the NA over which light returning to the collection system is collected and interferes with the reference light, generating signal. As an example, for a single mode fiber based OCT system, the collection system NA is the NA of the single mode fiber. Field illumination holoscopy systems have achieved resolutions beyond that given by the numerical aperture (NA) of the illumination beam by using a detector array to collect the light with a higher NA (see for example, PCT Publication No. WO 2015/189174, hereby incorporated by reference). However, this leads to a mismatch in the NA for illumination and collection, resulting in vignetting between the illumination and collection apertures outside the focal plane. Increased parallelization can reduce this vignetting, but the parallelization with current hardware reduces the scan speed for a single acquisition, leading to motion artifact issues.
The two approaches used previously for increasing the NA and therefore increasing the image resolution beyond that of the illumination beam have been: 1) simultaneously collecting light from the illumination region from multiple angles on different elements of the detector array, and 2) imaging the illumination area on to the detector array across multiple detectors so that sub-areas of the illumination region are resolved.
In both cases, the NA of the collection system in a field illumination OCT system was increased, creating a mismatch between illumination and collection NA, thus creating the vignetting issues as discussed above.
The importance of collecting angular diverse scattering information to achieve high transverse resolution in OCT is well recognized, and has been discussed in, for example, Fercher, A. F., et al. (2003) “Optical coherence tomography—principles and applications.” Reports on Progress in Physics 66: 239-203. It has previously been demonstrated that, after sequentially acquiring a set of closely spaced A-scans, one can combine the out of focus angular diverse scattering information from adjacent scans to remove out-of-focus-blur, generating depth invariant transverse resolution, where the resolution over an extended depth of field is given by the resolution of a single beam at focus (see for example, Davis, B. J., et al. (2008). “Interferometric Synthetic Aperture Microscopy: Computed Imaging for Scanned Coherent Microscopy.” Sensors (Basel) 8 (6): 3903-3931).
Sequential acquisition of OCT data at multiple angles has been described before, both for speckle reduction (see for example, Desjardins, A. E., et al. (2007). “Angle-resolved Optical Coherence Tomography with sequential angular selectivity for speckle reduction.” OPTICS EXPRESS 15 (10): 6200) and measurement of angle dependent scattering (see for example, Lujan, B. J., et al. (2015). “Directional Optical Coherence Tomography Provides Accurate Outer Nuclear Layer and Henle Fiber Layer Measurements.” Retina 35 (8): 1511-1520).
Simultaneous collection of multiple angles has been demonstrated for speckle reduction (see for example, Klein, T., et al. (2013). “Joint aperture detection for speckle reduction and increased collection efficiency in ophthalmic MHz OCT.” Biomed Opt Express 4 (4): 619-634) with multiple fiber optic collectors. However, the collection of light at different angles from the illumination limited the overlap region between the beams (vignetting outside the focal plane), resulting in the need to realign the relative positions of the collection beams for different eyes. Simultaneous collection of multiple angles has also been used for improving transverse resolution in partial or full field systems with a larger number of detector elements, which can overcome the vignetting issues by illuminating a large field of view (see for example, Hillmann, D., et al. (2012). “Common approach for compensation of axial motion artifacts in swept-source OCT and dispersion in Fourier-domain OCT.” Opt Express 20 (6): 6761-6776). However these systems have slow sweep speed due to the large number of acquisition channels, leading to motion artifacts as described in Hillmann.
Here, we present new techniques for OCT and holoscopy systems to achieve resolution beyond that given by the illumination beam or collection system through sequentially illuminating and imaging the sample at different angles. This allows high spatially invariant resolution while eliminating the vignetting issues of field illumination systems. While depth invariant resolution has been demonstrated previously, the potential to increase the imaging resolution of the system by sequentially scanning at different angles has not been recognized.